This invention relates to radiation detectors and more specifically to ionization-type radiation detectors employing liquid noble gas.
It will be understood by one skilled in the art that the term "radiation" as employed herein shall broadly include energetic particle radiation, such as electrons, alpha particles, or other heavy ions, as well as high energy electromagnetic radiation, such as x-rays or gamma rays. The term "detector" is broadly used to include devices capable of detecting radiation and may include means for measuring the energy and intensity of such radiation in a simple way or in a more complex way to form images.
Ionization-type detectors are well-known in the art. In a simple embodiment, an ionization-type detector may comprise a chamber containing a medium which may be a gas or a liquid. When radiation is introduced into the chamber and interacts with the molecules and atoms of the medium, the energy of the radiation is absorbed by the medium resulting in the production of ions. Generally, the number of ions produced will be proportional to the energy of the radiation. The ions are collected by means of a potential applied across electrodes within the chamber. The number of ions collected is related, inter alia, to the number produced in the medium and the strength of the electric field resulting from the voltage across the electrodes.
For the amount of collected charge to be a useful analog to the energy of the incident radiation, it is desirable that all or as much as possible of the energy of the incident radiation be absorbed through ionization of the medium. The number of molecules of the medium available to interact with incident radiation is clearly related to the physical size of the chamber as well as the density of the medium. It is preferable to use a medium with a higher density rather than increase the dimensions of the chamber because it results in an instrument that is more compact and has greater position resolution. Thus, in many applications it is desirable to use a high density liquid noble gas as a medium for an ionization detector. Liquid argon (LAr) for example, is a practical medium for calorimetry in high energy physics and liquid xenon (LXe) has been used for two-dimensional x-ray and gamma ray imaging devices.
In general, an ionization detector is referred to as an ionization chamber when the electric field is just large enough to collect the free charge produced by the ionizing events but not so large as to accelerate the free charge to the point that secondary ionization takes place. More sensitive detectors known as proportional chambers or proportional counters use a higher voltage to produce secondary ionization, effectively amplifying the collected charge. This effect is sometimes referred to as an avalanche process. Proportional amplification of collected charge has been practically achieved in gas filled detectors but has not been achieved in liquid noble gas detectors.
Another disadvantage in using liquid noble gases with certain highly ionizing radiation such as heavy ion beams is that although copious ions are produced when the energy of the radiation is deposited in the liquid noble gas, the high charge densities result in most of the free electrons being lost to recombination before they can be collected.
It has been suggested by T. Doke, PORTUGAL PHYS. 12 (1981)9, that a LAr detector for heavy ions may be constructed to obtain the advantage of the liquid noble gas medium, wherein charge would be collected and scintillation photons resulting from the recombination would be collected by separate detector means and the two signals added together to indicate the energy of the incident radiation. However, since scintillation photons are emitted in all directions, collecting all or a meaningful amount of the scintillation light or weighting the scintillation signals to account for inefficiencies in light collection is problematic.
Two classes of dopants have previously been used with liquid noble gas detectors. Molecules such as ethylene and methane have been used to cool excess electrons thus improving the charge collection time by reducing diffusion. This results in better energy resolution but actually reduces the amount of charge collected, which reduces the sensitivity of the measurement. This was reported by Shibamura, et al. Nucl. Instr. and Methods. 131 (1975)249. Xenon has been added to LAr to increase the ionization yield by converting excitons in the LAr into additional free charge. This approach, however, does not prevent the loss of free charge due to scintillation as discussed above. This approach was reported by Kubota et al. Phys. Rev. B13 (1976)1649.
Photosensitive materials have previously been used in the gaseous phase as a medium in a secondary ionization detector used to detect ultraviolet scintillation photons emitted from the medium in a primary detector employing noble gases in a liquid or gaseous phase. See for example, A. Polycarpo, Nucl. Instr. and Methods, 196, (1982) 53 or U.S. Pat. No. 4,429,228, issued Jan. 31, 1984, to David F. Anderson.
Thus, it is an object of the present invention to provide a method of detecting radiation using a liquid noble gas ionization detector with increased sensitivity and increased energy resolution. It is another object of the invention to provide a method of converting scintillation photons to collectable free charge in a high energy ionization-type radiation detector. It is yet another object of the invention to provide a liquid xenon, ionization-type x-ray detector in which practical proportional amplification of collected charge is achieved. Additional objects, advantages, and novel features of the invention will be set forth in part in the following description and will be readily apparent to those skilled in the art.